The present invention relates to a semi-synthetic process to convert a naturally occurring taxane into a suitable starting material for the synthesis of paclitaxel and related compounds. Specifically, the present invention relates to a process for the conversion of 9-dihydro-13-acetylbaccatin III into a 7-protected baccatin III which can then be used as starting material for the synthesis of taxane derivatives such as paclitaxel, docetaxel, cephalomannine and other taxanes structurally related to baccatin III. The method as described uses a preparative scale technique which is amenable to commercial scale-up.
The taxane family of terpenes is considered to be an exceptionally promising group of cancer chemotherapeutic agents. Many taxane derivatives, including paclitaxel, docetaxel, taxcultine canadensol are highly cytotoxic and possess strong in vivo activities in a number of leukemic and other tumor systems. Paclitaxel, and a number of its derivatives, have been shown to be effective against advanced breast and ovarian cancers in clinical trials (W. P. MacGuire et al., Annals of Internal Medicine, vol 111, pg. 273, 1989). They have also exhibited promising activity against a number of other tumor types in preliminary investigations. Paclitaxel has recently been approved in the U.S. and Canada for the treatment of ovarian cancers (Rose et al., in xe2x80x9cThe Alkaloidsxe2x80x9d, A. Brossi, Ed., Academic Press, New York, Paclitaxel: A Review of its preclinical in vivo Antitumor Activity. Anti-Cancer Drugs 3, 311-321 1992; and Suffness, M., Paclitaxel: from discovery to therapeutic use. Ann. Rep. In Med. Chem., 28,305-314, 1993). Taxanes are believed to exert their antiproliferative effect by inducing tubulin polymerization, which forms extremely stable and nonfunctional microtubules (Schiff, et al., Promotion of Microtubule Assembly in vitro by Paclitaxel. Nature, 277, 665-667, 1979). However, a major problem with the clinical studies is the limited availability of paclitaxel and its derivatives.
Taxanes are natural products which can be isolated from yew trees. The first taxane to be characterized was paclitaxel (also known as taxol(trademark)) which was isolated and purified from the bark of the Pacific yew in 1971. The only available natural source of paclitaxel to date are several species of a slow growing yew (genus Taxus), wherein paclitaxel is found in very low concentrations (less than 400 parts per million) in these trees. Furthermore the extraction is difficult, tile process is expensive and the yield of paclitaxel is low (Huang et al, J. Nat. Prod. 49 665, 1986, reported a yield of 0.00025% of a crude paclitaxel fraction from Taxus brevifolia bark). 
Paclitaxel can be isolated from the bark of Taxus brevifolia, the pacific yew tree, or from Taxus baccata, its European relative. Since removal of the bark destroys the trees and endangers the species, isolation of taxanes from the stems and needles of various Taxus species offers hope that the supply of taxanes, in particular paclitaxel, would become more abundant.
The preparation of paclitaxel derivatives, some of which have been reported to demonstrate enhanced chemotherapeutic activity, ultimately depends upon the supply of the parent compound-baccatin III. The structure of baccatin m has the basic diterpenoid structure of paclitaxel without the side chain at the C-13 position. 
Baccatin III is an important staring material in paclitaxel semi-synthesis. Therefore the significance of baccatin III will likely increase as more clinical studies are performed using paclitaxel. One such reason is that it appears that water soluble paclitaxel-like compounds with slightly modified C-13 side chains may be more desirable as cancer chemotherapeutic agents than the naturally occurring less water soluble paclitaxel. This increases the urgent need for baccatin III as a starting material to synthesize both paclitaxel and second or third generation paclitaxel-like compounds. There is, therefore, a need for an improved method of isolating and/or synthesizing Baccatin III.
The majority of research to date has been concerned with the development of techniques to increase the availability of either paclitaxel or baccatin III. These techniques have included improvements to the isolation and purification processes (U.S. Pat. No. 5,407,674 and U.S. Pat. No. 5,380,916), to the total synthesis (U.S. Pat. No. 5,405,972) and partial synthesis (from more abundant paclitaxel precursors) and also isolation from a variety of cell culture systems (U.S. Pat. No. 5,019,504). In Addition, an endophytic fungi isolated form bald cypress (Taxodium distichum) was reported to produce very small amounts of paclitaxel (Strobel, R. et al., Microbiology, 142, 2223-2226, 1996)
Because of the structural complexity of paclitaxel, partial synthesis is a far more viable approach to providing adequate supplies of paclitaxel and paclitaxel precursors than total synthesis. The first successful semi-synthesis of paclitaxel was developed by Denis et al, (U.S. Pat. No. 4,924,011 re-issued as U.S. Pat. No. 34,277), using the starting material 10-deacetylbaccatin III which can be extracted in relatively high yield from the needles of specific species 
In fact, most of the research to date regarding the semi-synthesis of paclitaxel has involved 10-deacetylbaccatin III. The conversion of 10-deacetylbaccatin III into paclitaxel is typically achieved by protecting the hydroxy at C-7, attachment of an acetyl group at the C-10 position, attachment of a C-13 xcex2-amido ester side chain at the C-13 position through esterification of the C-13 alcohol with the xcex2-lactam moiety, and deprotecting C-7. Since the supply of 10-deacetylbaccatin III is limited, other sources should be pursued.
Research has recently centred on semi-synthesis of paclitaxel from 10-deacetylbaccatin III because it is the major metabolite obtained from specific species of the European Yew (Taxus baccata). However to date, the yields of 10-deacetylbaccatin III have been unsatisfactory, ranging from 50-165 mg taxane per kilogram of starting material (i.e. providing yields of between 0.005 to 0.017%). Hence there is an urgent need for novel semi-synthetic techniques to produce higher yields of paclitaxel precursors, such as baccatin III, for subsequent use in the production of paclitaxel derivatives. The present invention provides such a method, describing the conversion of a known taxane (9-dihydro-13-acetylbaccatin III), which is produced as a major metabolite in a certain species of taxus, into a paclitaxel precursor which produces relatively large amounts of a 7-protected baccatin III. Depending on the collection sites, the yield of 9dihydro-13-acetylbaccatin III can vary from 2.0 to 2.5 g per kilogram of dry plant and this taxane can be chemically transformed, by the present invention, into 7-protected baccatin III in 20% yield.
The present invention is directed towards a new method of producing a 7-protected baccatin III, from a naturally occuring taxane (9-dihydro-13-acetylbaccatin III) which is produced in high yields in Taxus canadensis. The 7-protected baccatin III can be used as a starting material for the synthesis of paclitaxel and paclitaxel derivatives.
Accordingly, it is an object of this invention to provide a reproducible method for the semi-synthesis of 7-protected baccatin III from the naturally occurring compound, 9-dihydro-13-acetylbaccatin III, isolated from plant matter derived from the Taxus genus of plants.
It is a further object of this invention to provide a method for the semi-synthesis of baccatin III, and other protected intermediates, that proceeds with higher yields than currently known methods.
Still a further object is to provide a simple, inexpensive method of preparing 7-protected baccatin III that proceeds at room temperature.
It is also an object of this invention to provide a method for the semi-synthesis of 7-protected baccatin III, from plant matter that is on a preparative scale which is amenable to commercial scale-up processes.
The present invention provides a process for the preparation of a taxane of formula (III) 
which comprises the steps of: (i) protecting a hydroxy group at the 7-position of 9-dihydro-13-acetylbaccatin III; (ii) oxidizing a hydroxy group at the 9-position of 7-protected 9-dihydro-13-acetylbaccatin; (iii)deacylating an ester at the 13-position to form a 13-hydroxy compound of formula III, wherein P is a hydroxy protecting group. The present invention provides an additional step of removing the hydroxy protecting group P at the 7-position to yield baccatin III.
The present invention provides a process for the preparation of a 7-protected-9-dihydro-13-acetylbaccatin of formula I. 
wherein P is a hydroxy protecting group, which comprises the step of reacting 9-dihydro-13-acetylbaccatin III with a hydroxy protecting group to form a compound of formula I.
The present invention also provides a process for the preparation of a compound of formula II 
which comprises the step of oxidizing a compound of formula I.
The present invention further provides a process for the preparation of a compound of formula III from a compound of formula II 
wherein P is a hydroxy protecting group, which comprises converting a 13-acetyl group to 13-hydroxyl group of a compound of compound of formula II.
In a preferred embodiment 7-protected-9-dihydro-13-acetylbaccatin is formed by reacting 9-dihydro-13-acetylbaccatin III with a silylhalide, benzylhalide or alkylhalide, the halide is selected from Cl, Br, or I. Preferred protecting reagents are t-butyldiphenylsilylchloride, t-butyldimethylsilylchloride, triethylsilylchloride or triisopropylsilylchloride.
In a preferred embodiment the oxidation is facilitated by Jones"" reagent, pyridinium dichromate, a Swern oxidation, a permanganate ion or Sarret""s reagent.
In a preferred embodiment deacylation is facilitated by reaction with an alkylalkalimetal or arylalkalimetal reagent. Most preferred regent for deacylation is n-butyllithium.
These and other objects, as well as the nature, scope and utilization of this invention, will become readily apparent to those skilled in the art from the following description, the drawings and the appended claims.